1. Field of the Invention
The present invention relates generally to gas turbine engine, and more specifically for an air cooled turbine blade.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils. Typically only the first and second stages of blades and vanes are cooled while the later stage or stages are uncooled because the gas stream temperature has decreased below the maximum acceptable temperature for these airfoils. As turbine inlet temperatures rise, even these later stage airfoils will require cooling, if not to prevent melting of the material, but also to prevent thermal fatigue or creep issues.
A prior art industrial gas turbine engine includes stator vanes and rotor blades made from high strength and high temperature resistant materials in order to withstand the high temperatures and, in the case of the turbine rotor blades, high centrifugal loads. A typical material is a nickel super alloy that can be formed by investment or lost wax casting, or for blades that will have stronger tensile stress capability from a directionally solidified or single crystal material. However, to allow for operating temperatures above these materials melting temperature, complex airfoil cooling is required. Recent industrial gas turbine engines operate with a turbine inlet temperature of around 2,600 degrees F. (1427 degrees C.). Airfoil cooling can include internal convection and impingement cooling and external film cooling to draw heat away from the airfoil and to provide a protective blanket or layer of cooling air to the external airfoil surface.
The pressurized cooling air required for cooling these airfoils typically is bled off from the compressor. In an engine where several blade rows must be cooled, the amount of cooling air flow can be as high as 25-30% of the entire engine air flow. The cooling air from the compressor is not used to produce any work in the engine and therefore the work by the compressor that goes into compressing the cooling air is wasted.
Nickel based alloys have been used for turbine blades and vanes because of the stronger material properties and high temperature resistance. Several metallic alloys exist that have great strength and creep properties at very high temperatures (greater than 4,000 degrees F.) when compared to nickel based alloys. However, these materials have poor oxidation resistance and therefore have not been used in turbines. When exposed to the combustion gas flow that typically includes unburned oxygen, these materials quickly oxidize away. These materials include refractory metals such as Niobium (Columbium), Molybdenum, Tungsten, Rhenium and Tantalum, and oxidation dispersed strengthened (ODS) alloys. Protective coatings have been used to protect the base alloy but with limited success.